Every time you read a chemical label, you are looking at a universal code that scientists use to describe the entire universe. A chemical symbol is a one- or two-letter notation used to represent an element. The first letter is always uppercase, and the second is always lowercase (e.g., for helium, never ).
For your OCR exam, you must recognise the names and symbols of the first 20 elements, alongside key groups:
Elements exist in different forms depending on their group. Noble gases are monatomic, meaning they exist as single, unbonded atoms that do not form ions in standard contexts. In contrast, halogens and a few other non-metals are diatomic, existing as molecules made of two atoms chemically bonded together ().
Transition metals are found in the central block of the Periodic Table. They have high densities, variable oxidation states (can form ions with different charges), and typically form coloured compounds. Common examples include iron (), copper (), and zinc (). Other common elements frequently appearing in OCR papers include silver () and the Group 2 metal barium ().
A chemical formula represents a substance using symbols and subscripts to show the exact ratio of atoms. To deduce the formula of an ionic compound, you must know the valency of each element. Valency is the "combining power" of an atom, linking directly to the electrons in its outer shell and its Periodic Table group.
Transition metals often have their valency indicated by a Roman numeral (e.g., Copper(II) means ). You must also memorise common polyatomic ions: hydroxide (), nitrate (), sulfate (), carbonate (), and ammonium ().
Calculate the chemical formula for aluminium sulfate using the cross-over method.
Step 1: Write the symbols for the elements or polyatomic ions.
Step 2: Write the valency (charge magnitude) below each symbol.
Step 3: Cross over the valency numbers to become the subscripts for the opposite symbol.
Step 4: Write the final simplified formula. Brackets are essential because there is more than one polyatomic ion.
The Law of Conservation of Mass states that mass is never lost or gained in a chemical reaction. Atoms are simply rearranged to form new products, so the total mass of the reactants always exactly equals the total mass of the products.
In a closed system, no substances can enter or leave, meaning the measured mass remains visibly constant throughout the reaction. However, in an open system, gases can enter from the atmosphere or escape into the surroundings. This can cause the measured mass on a balance to apparently increase (if a gas reacts with a solid) or decrease (if a product gas escapes).
To obey the conservation of mass, we write a balanced chemical equation. This ensures the number of atoms for each element is identical on both sides of the arrow.
We achieve this by adding stoichiometric coefficients — large numbers placed in front of a formula. You must never change the small subscript numbers inside a chemical formula, as this would alter the molecular structure and completely change the identity of the substance.
Write a balanced chemical equation for the reaction between lithium and oxygen to form lithium oxide.
Step 1: Deduce the formulae of the reactants and products using valency.
Step 2: Write the unbalanced equation.
Step 3: Count the atoms on both sides.
Step 4: Balance the oxygen by placing a coefficient of 2 in front of lithium oxide.
Step 5: Balance the lithium by placing a coefficient of 4 in front of lithium.
State symbols are small letters in brackets placed after a chemical formula to indicate its physical state at the time of the reaction. The four symbols are for solid, for pure liquid, for gas, and for aqueous (dissolved in water).
A common reaction involving aqueous solutions can produce a precipitate, which is an insoluble solid formed when two solutions are mixed.
In this equation, the identifies silver chloride as the solid precipitate, while the other substances remain fully dissolved in the water.
Students often change the small subscript numbers inside a chemical formula to balance an equation (e.g., changing to ). You must only change the large stoichiometric coefficients in front of the formula.
In exam questions, a 'balancing mark' is almost always dependent on a 'formulae mark'. If you deduce the chemical formula incorrectly, you cannot score the subsequent marks for balancing the equation.
Do not confuse liquid (l) with aqueous (aq); only use (l) for pure liquid substances like molten salts or pure water, and use (aq) for any acid or substance described as a solution.
When explaining a decrease in mass during a reaction in an open system, state that the gas 'escaped into the surroundings' rather than saying mass was 'lost'.
Chemical symbol
A one- or two-letter internationally agreed code used to represent an element, always written in upright roman text (e.g., Na).
Alkali metals
The elements in Group 1 of the Periodic Table that react violently with water and form 1+ ions.
Halogens
The highly reactive non-metals in Group 7 of the Periodic Table that form 1- ions and exist as diatomic molecules.
Noble gases
The extremely unreactive elements in Group 0 of the Periodic Table that have full outer electron shells and exist as monatomic atoms.
Monatomic
Elements that exist as single, unbonded atoms rather than molecules.
Diatomic
Elements that exist as molecules composed of two chemically bonded atoms, such as O₂ or Cl₂.
Transition metals
Metals found in the central block of the Periodic Table that have high densities, variable oxidation states, and form coloured compounds.
Chemical formula
A representation of a substance using chemical symbols and subscripts to show the ratio of atoms of each element present.
Valency
The combining power of an element, determined by the number of electrons it needs to lose, gain, or share to achieve a full outer shell.
Law of Conservation of Mass
The fundamental principle that mass is never created or destroyed in a chemical reaction; the total mass of reactants equals the total mass of products.
Closed system
A chemical reaction environment where no substances can enter or leave, keeping the measured mass entirely constant.
Open system
A chemical reaction environment where gases can enter from the atmosphere or escape into the surroundings, causing apparent mass changes.
Balanced chemical equation
A reaction representation where the number of atoms for each element is exactly equal on both the reactant and product sides.
Stoichiometric coefficients
The large numbers placed in front of chemical formulae to balance an equation, which multiply every atom in that formula.
State symbols
Small bracketed letters (s, l, g, aq) placed after a formula in an equation to denote the physical state of a substance.
Precipitate
An insoluble solid that forms when two aqueous solutions react together.
Put your knowledge into practice — try past paper questions for Chemistry A
Chemical symbol
A one- or two-letter internationally agreed code used to represent an element, always written in upright roman text (e.g., Na).
Alkali metals
The elements in Group 1 of the Periodic Table that react violently with water and form 1+ ions.
Halogens
The highly reactive non-metals in Group 7 of the Periodic Table that form 1- ions and exist as diatomic molecules.
Noble gases
The extremely unreactive elements in Group 0 of the Periodic Table that have full outer electron shells and exist as monatomic atoms.
Monatomic
Elements that exist as single, unbonded atoms rather than molecules.
Diatomic
Elements that exist as molecules composed of two chemically bonded atoms, such as O₂ or Cl₂.
Transition metals
Metals found in the central block of the Periodic Table that have high densities, variable oxidation states, and form coloured compounds.
Chemical formula
A representation of a substance using chemical symbols and subscripts to show the ratio of atoms of each element present.
Valency
The combining power of an element, determined by the number of electrons it needs to lose, gain, or share to achieve a full outer shell.
Law of Conservation of Mass
The fundamental principle that mass is never created or destroyed in a chemical reaction; the total mass of reactants equals the total mass of products.
Closed system
A chemical reaction environment where no substances can enter or leave, keeping the measured mass entirely constant.
Open system
A chemical reaction environment where gases can enter from the atmosphere or escape into the surroundings, causing apparent mass changes.
Balanced chemical equation
A reaction representation where the number of atoms for each element is exactly equal on both the reactant and product sides.
Stoichiometric coefficients
The large numbers placed in front of chemical formulae to balance an equation, which multiply every atom in that formula.
State symbols
Small bracketed letters (s, l, g, aq) placed after a formula in an equation to denote the physical state of a substance.
Precipitate
An insoluble solid that forms when two aqueous solutions react together.